1. Field of the Invention
The present invention relates to an improved polyethylene modified asphaltic composition and its method of preparation.
2. Discussion of Related Art
Asphalt is a bituminous material resulting from the distillation of crude oil. Typically, asphalt is derived from the bottoms of a vacuum distillation tower and has an atmospheric boiling point of at least 400.degree. C. Because it is hydrophobic and has good adhesiveness and weatherability, asphalt has been used widely as a binder in paving materials and as a coating for roofing shingles.
When used in roofing applications (e.g. a shingle coating or a saturant), the asphalt is usually polymerized by undergoing oxidative dehydrogenation in which the asphalt is contacted with an oxygen-containing gas (usually air) at temperatures ranging between about 200.degree. C. and about 300.degree. C. for between about 6 and about 24 hours. This procedure is described in detail by L. W. Corbett in Bituminous Materials: Asphalts, Tars and Pitches, Interscience Publishers, New York (1965) and by E. J. Barth in Asphalt Science and Technology, Gordon and Breach Science Publishers, New York (1968), the disclosures of each being incorporated herein by reference. Oxidative dehydrogenation improves the creep (i.e. flow) resistance and weatherability of the asphalt, and reduces its sensitivity to temperature changes.
When used as a conventional paving binder, the asphalt does not usually undergo oxidative dehydrogenation, but rather, is manufactured to meet certain control specifications such as flash point (ASTM D-92), penetration at 25.degree. C. (ASTM D-5), apparent viscosity at 60.degree. C. (ASTM D-2171) and kinematic viscosity at 135.degree. C. (ASTM D-2170). In addition to the control specifications, a paving asphalt should also meet certain performance specifications such as ductility (ASTM D-113), solubility in trichloroethylene (ASTM D-2042), and thin film oven aging (ASTM D-1754).
In the past, asphalts which met the above-mentioned control and performance specifications were generally considered more than adequate as paving binders. However, more recently, asphalt users have been demanding higher performance products. For example, today's increased heavy truck traffic volume is stressing and destroying roads. One of the major problems resulting from this increased traffic loading is pavement rutting due to the natural tendency of asphalt to creep over a period of time, particularly during the increased temperatures of summer. Therefore, road construction contractors are now in serious need of paving asphalt binders which have an increased resistance to creep.
As a further example of the increasing demand for enhanced performance, low temperature cracking in roads and roofing products is still a major problem. Accordingly, a reduction of the temperature at which the product will crack (typically below about 0.degree. C.) would provide an additional benefit throughout the construction industry. A formulation with improved low temperature cracking properties would have higher tensile strength to resist the accumulated tensile stresses and reasonably similar or lower stiffness to minimize brittleness and fracture.
One approach for changing or adjusting the rheological and chemical properties of asphalt is to incorporate certain additives therein. For example, a polymer such as polyethylene has been added to asphalt to produce asphalts which exhibit an increased resistance to creep or deformation. In addition, polyethylene modified asphalts must also have good compatibility between the asphalts and polyethylene, and be stable at high temperatures during storage and asphalt plant operation for ease of handling and application. Compatibility between the polyethylene and asphalt is very important to ensure that the engineering properties of both the asphalt and polyethylene are transferred to the finished product and to provide good long term performance. However, most polyethylene modified asphalts require either continuous mixing or the addition of various additives to avoid phase incompatibility (See for example U.S. Pat. Nos. 4,240,946 and 4,314,921 which require continuous high shear mixing to obtain physical dispersion of a polyolefin in bitumen, and Jew et al., Journal of Applied Polymer Science, 31, pp. 2685-2704 (1986) which discloses the addition of one or more dispersants to a polyethylene modified asphalt to avoid phase separation, the disclosures of each being incorporated herein by reference. See also Transportation and Road Research Laboratory Report 1101 by Denning, J. H. et al., Highways and Structures Department, Crowthorne, Berkshire, England (1983)).
Polyethylene has also been added to asphalt containing blends for other purposes. For example, European Patent Application 130,367 discloses thermoplastic moulding compounds which contain an ethylene copolymer having a vinyl ester and/or an alkyl ester, bitumen, carbon black, glyceryl monostearate, filler material and a second ethylene copolymer which is described as a linear low density polyethylene having a melt index from 0.1-10 g/10 mins. and a density between 0.918-0.935 g/cm.sup.3. The combination of the two ethylene copolymers with bitumen forms a bituminous moulding compound having high strength at room temperature, high dimensional stability under heat and rigidity. As another example, European Patent Application 64,630 discloses a flame retardant halogen-free thermoplastic moulding composition which contains ethylene copolymer, bitumen, aluminum hydroxide, red phosphorus, carbon black, glyceryl monostearate and, if necessary, polyethylene having a melt index of 0.1-10 g/10 min. and a density from 0.917 to 0.940 g/cm.sup.3. The dimensional stability of the mixture under heat is increased due to the polyethylene addition. Both patentees disclose that the moulding compounds formed are suitable for the preparation of moulded articles, particularly sealing widths for structural and civil engineering applications. In addition, both patentees use asphalt as a diluent rather than as a major component of the product.
Accordingly, none of the foregoing references teach or suggest a polyethylene modified asphalt composition (and a method for making same) which does not require continuous mixing or addition of dispersants or other compounds to maintain phase compatibility of said composition.